Motto: "O dark dark dark. They all go into the dark,
The vacant interstellar space,
the vacant into the vacant"
(T. S. Elliot, in "East Coker", No. 2 of The Four Quartets)

1. INTRODUCTION

Cosmology is the study of the universe as a whole, it is the science
of the large-scale structure of the universe, its origin, and its
evolution from the early times into the future. In this context
universe means all that exists in a physical sense, not only the
part of the universe that we can observe
(Ellis 2007)
using our telescopes and detectors on the ground and in space. The
observable universe could certainly be a tiny fraction of the whole
universe, even an infinitesimal fraction if the universe were infinite.

Cosmology today can be considered a branch of physics with a slight
difference: we cannot experiment with the subject of our discussion,
the universe, we can only observe it and model it.

The statement that "we are living in the era of precision
cosmology" is certainly one of the most heard ones in the last 10
years in conferences, seminars and talks about the field. There is
no doubt that within this period, modern cosmology has expanded from
what
Allan Sandage
(1970)
once described as "the search for two numbers",
meaning the Hubble and the deceleration parameters. These and a
few more numbers are conforming now a self-consistent set, derived
from several different cosmological observations: high-redshift
supernovae, fluctuations of the Cosmic Microwave Background (CMB)
radiation, the large-scale structure of the universe, gravitational
lensing, etc., but the emergent concordance cosmology provided by
all these probes (sharing all beautiful hard-to-get cosmological
data) is in a sense disappointing. We need to claim for the
existence of gravitating non-baryonic dark matter of unknown nature,
and furthermore, the universe today has to be dominated by an exotic
dark energy, acting as a repulsive gravity. Some cosmologists take
those theories as seriously as Ptolemy and colleagues took epicycles
and deferents to reconcile the geocentric model with the early
observations of planetary motion, or how physicists prior to the
Michelson-Morley experiment considered the aether of undoubted
certain existence. Since cosmology is not an experimental science,
but an observational one, we must take this into account when we try
to falsify our theories in the sense advocated by
Karl Popper
(1959):
our requirement is just that our theories should be consistent with
present and future observations. In contrast to what happens in
experimental physics, astronomers cannot modify the object under
study. They cannot control it in any way; they only can observe it
many times, with different exposure times or at different
frequencies or observe many objects of the same type
(Kolb 2007a).
This fact inevitably conditions the way we do our
research and plan our observations.